This invention relates to a method and apparatus for raising or pumping liquids wherein the pump itself has no moving mechanical parts. In another aspect, it relates to a method and gas lift pump for raising liquids to a height more than twice the submergence depth of the pump. The invention in yet another aspect relates to a method and submergence gas lift pump for raising corrosive liquids.
Many industrial processes include the use of corrosive liquids as an agent in the process or result in corrosive liquids as a product or by-product. Equipment must be suitable for pumping or transferring such corrosive liquids from one vessel or location to another, such as within a plant facility. Corrosion resistant mechanical pumps can be designed specifically for pumping such corrosive liquids; however, such mechanical pumps have disadvantages. For example, packing materials within the pump must be able to withstand the corrosive liquids and may be required to be stable at high temperatures, e.g., such as in excess of 700.degree. C. Such mechanical pumps also have multiple moving parts which must be resistant to the corrosive liquids being pumped. Such features of mechanical pumps add significantly to the cost of manufacturing and of repairing or servicing. As an alternative to mechanical pumps, gas lift pumps having no moving mechanical parts can be used to avoid some of the disadvantages of mechanical pumps.
Conventional gas lift pumps work on a gas lift principle of introducing finely divided gas bubbles into a lift tube to reduce the apparent density of a liquid contained in the lift tube. The liquid (and gas) in the lift tube and a separate body of "parent" liquid, e.g., liquid in a pressure-head-developing standpipe, each act as two arms of a manometer which are of unequal density because of the density change in the lift tube attributable to the addition of gas bubbles in the liquid. A flow of liquid in the manometer ensues up the tube in the direction of the liquid having the gas bubbles contained therein when the product of the parent liquid density multiplied times the depth of submergence, e.g., the height of liquid in a standpipe used to develop the pressure head, is greater than the product of the apparent liquid density multiplied times the length of the lift tubes. Such conventional gas lift pumps alone, i.e., incorporating no more than the gas lift principle, cannot lift liquids to a height significantly greater than the submergence depth of the pump. By "submergence depth" is meant an effective height of liquid which acts to develop and apply a pressure head on the lower end of the lift tube. The effective height of liquid may be provided by liquid contained in a standpipe, in a pool surrounding the lift tube, or by other forms of a pressure-head-developing column of liquid.
Use of the gas lift principle may include introducing small bubbles of the lifting gas directly into the lift tubes, as in U.S. Pat. No. 1,921,060, which discloses a method of purifying metals using reducing gas or inert gas rising through a tubular member filled with impure metal. The rising small bubbles of gas act not only to purify but also to lift the metal to a chamber in which a vacuum is applied. The patent further discloses that a plurality of tubes may be used to increase the rates of treating and of raising the metal by using a multiplicity of tubes operating in parallel. A cascade system is disclosed using multiple tubes operating independently but in series flow.
U.S. Pat. No. 2,399,634 relates to a method of pumping molten metal by submergence gas lift using two connected columns of metal. One column of metal flows downwardly, and the second column of metal flows upwardly in response to a decreased apparent liquid density attributable to rising small gas bubbles entering the second column.
A method and apparatus of degassing molten metals continuously into a vacuum chamber is shown in U.S. Pat. No. 2,893,860. Insoluble gases are injected as small bubbles into a conduit of molten metal to carry the molten metal upwardly through the conduit to the vacuum chamber where the carrying gas and undesired gases are separated, permitting the molten metal to flow back into the original chamber.
Another method of introducing small bubbles of lifting gas directly into lifting tubes is shown in U.S. Pat. No. 3,033,550. A method and apparatus for vacuum treating during degasification of molten metal are disclosed wherein a gas inlet pipe having a plurality of nozzles is disposed below the opening of the rising pipe or lifting tube. A current of gas having a state of fine subdivision is disclosed to be preferred over gas bubbles as large as the inside diameter of the lifting pipe.
A lift pump shown as having bubbles of lifting fluid as large as the inside diameter of the lifting pipe is disclosed in U.S. Pat. No. 1,741,571, which relates to a liquid raising apparatus having a chamber with a lower bowl-like portion and with an upper portion divided by a baffle to form two branches. The lower extremity of the baffle extends horizontally across the chamber. Water enters the chamber through a lower port and rises upwardly until it reaches the lower extremity of the baffle, where it acts to seal air in one branch, which air enters the branch from an inlet port. The apparatus operates by increasing air pressure sufficiently through the air inlet port to cause slugs of water to blow upwardly through the other branch and out through a port. The water slugs are disclosed to be impelled upwardly by the air bubble such that intervals of air flow provide alternate slugs of water and slugs of air rising upwardly through the pipe.
U.S. Pat. No. 4,135,364 discloses an air lift pump energy conversion apparatus having a plurality of vertical lift tubes completely immersed in water. The side by side tubes discharge water upwardly within a hood to drive a small turbine-like motor. Each lift tube has a shorter, smaller diameter inner tube concentric therein and separated from the tube by a horizontal annular fluid-tight partition. Compressed air is introduced into the space between the tubes and below a partition to form a coherent bubble for forcing water into the inner tube. The air then enters the inner tube and passes upwardly into the lower end of the lift tube as a coherent air bubble shown as extending completely across the interior diameter of the lift tube.
U.S. Pat. No. 532,699 discloses a process and apparatus for elevating liquids having a submerged pipe with gas injected at the base, a la the principle used in U.S. Pat. Nos. 1,921,060; 2,399,634; and 2,893,860 presented above, to lift a liquid an additional height less than or equal to the pipe's depth of submergence. A cascade system as mentioned in U.S. Pat. No. 1,921,060 is disclosed.
French Pat. No. 801,935 discloses a liquid elevator device having a lower housing communicating with the lower part of a plurality of conduits which also communicate with an upper housing. Liquid is boiled in each tube to form vapor bubbles in situ along the entire length of the liquid column.
Methods and apparatus incorporating the gas lift principle can be useful for pumping corrosive liquids; however, conventional gas lift pumps have significant problems and drawbacks. Simplicity of construction is an important feature in gas lift pumps especially when the pumps are used for handling corrosive liquids. Conventional lift pumps either lack such simplicity or provide only poor efficiency based on the volume of liquid lifted per volume of lifting fluid injected into the pump. Conventional lift pumps can lift liquids only to a height not significantly greater than the submergence depth of the pump. Conventional lift pumps further lack adequate control of the flow of lifting fluid and the liquid being raised by the pump so as to function as a combined valve and pump capable of regulating the flow rates of liquid and lifting fluid.